Measles virus (MV) is a leading cause of child mortality in developing countries despite the availability of a live attenuated vaccine for over 40 years. Severely immune-compromised people are particularly at risk for MV. The most serious manifestations of MV infection, including encephalitis, occur in people with impaired cellular immunity. MV affects the central nervous system (CNS) in up to half of routine cases; with adequate cellular immunity the infection is eradicated, but individuals with impaired cellular immunity are at a disadvantage. In a recent MV outbreak in South Africa several people died of MV CNS infection. We analyzed the viruses from these patients and found that specific intra-host evolution of the MV fusion machinery -- receptor binding protein (H) + fusion protein (F) -- had occurred. Normally, the MV F is synthesized and maintained in a pre-fusion state until it reaches the cell surface, and this pre-fusion state is intrinsically unstable and thermodynamically driven to the post-fusion state in a process that requires a signal from H upon H's interaction with receptor. However in the case of the ?CNS-adapted? viruses, a mutation in F allows it to promote fusion with less dependence on interaction of H with the two known MV cellular receptors; this F is activated independently of H or receptor. The CNS isolate F represents an ideal target for identifying small molecules that block the spread of MV in the CNS by destabilizing the F protein, and thereby promoting premature folding of F to its post-fusion state. Such compounds will effectively decrease the amount of pre-fusion F that is available to mediate cell- to-cell fusion, ultimately halting spread of MV in the CNS. We have adapted and validated a cell based bioluminescence based High Throughput Screen (HTS) assay. The Columbia University Medical Center HTS facility library will be screened for small molecules that efficiently block the fusion mediated by the F from the CNS isolate. Aim 1: Primary HTS of small molecule libraries will be performed. Orthogonal screenings in cell lines and human neurons will confirm efficacy against live virus. Aim 2: The mechanism of action of selected small molecules will be assessed using specific functional assays. Ex vivo efficacy studies will assess the antiviral activity of small molecules in relevant tissues. Viral evolution studies under the selective pressure of small molecules will determine the potential for emergence of viral resistance.